This competitive renewal application will further challenge our hypothesis that mitochondrial (mtDNA) defects are important contributors to hypertension (HTN). During the present funding period, we developed a novel method for the detection of mitochondrial involvement in complex disorders and applied it to the hypertension data set from 350 Caucasian and 98 African American pedigrees ascertained through the HTN clinics at Boston Medical Center and collaborating clinical sites. Remarkably, we found the fraction of families potentially due to mtDNA mutations to be approximately 55% (95% Cl, 45%-65%). Sequence analysis of the entire mitochondrial genome in 10 African American and 10 Caucasian hypertensive probands from families with suspected mitochondrial involvement led to the identification of several novel, as well as previously reported, mutations with a likely pathogenic effect. Molecular biological characterization of two mutations, T3308C in the ND1 gene and T5655C in the tRNA-Ala gene, detected in African Americans of Lib haplogroup, revealed a reduced Complex I activity in the mitochondrial cybrids containing these mutations. Preliminary data from case-control association study of another mutation, the A10398G in the ND3 gene, conducted in 700 unrelated individuals provided evidence for association of the 10398G allele with HTN. In the present application, we seek to confirm our findings in an independent cohort and to further assess the role of mtDNA variants in blood pressure (BP) homeostasis. Our specific aims are: 1) confirm maternal effect on BP inheritance in Framingham Heart Study participants using existing clinical and genetic data sets from 330 largest Framingham families;2) identify rare mtDNA variants with potential effects on BP by sequencing the mitochondrial genome from individuals with extreme systolic (n=50) and diastolic (n=50) BP phenotypes;3) test the association of selected mtDNA variants with HTN and related BP phenotypes by case-control and quantitative association analyses.